Methods are known for forming a metal article from a metal foam and one or more metal components. Often the metal foam is contacted or joined to the metal component or components by a conventional bonding technique such as brazing, welding, soldering and crimping.
It is understood by those skilled in the art that brazing is the joining of similar or dissimilar metals with a filler metal or alloy of lower melting point than that of the metals being joined. A brazing metal or alloy must have high fluidity and the ability to “wet” or alloy with the metals being joined when at the brazing temperature. For certain applications, brazing has many advantages over other joining methods, including high resistance to vibration and impact and a strong bond in high-temperature service. Additionally, brazing is an optimal joining technique for articles that will be used in high-pressure service because of the small clearances between the metals to be joined and the deep penetration of the brazing metal or alloy into the joint. A brazing metal or alloy may be provided in many forms, including, but not limited to, a paste, a spray, wires and foils.
In a known method, metal foam is contacted to or joined with a metal component using a braze metal in the form of foils. For example, in a known method, a cylinder of polymeric foam that has been impregnated with a slurry coating of powdered metal and thermally-treated to volatilize the polymeric foam and sinter the metal is surrounded by a braze metal foil and inserted into a solid metal component such as a tube. The entire assembly is then placed into a sintering furnace and thermally-treated to braze the metal foam and metal tube together.
However, there are problems associated with this and many other known methods of brazing. For example, with a metal foam in a metal tube, one such problem is that upon thermally-treating, the polymeric foam is volatilized leaving behind a metal foam structure composed of metallic ligaments or struts that are in contact with the metal tube. Since the metal foam is not solid, but rather a web-like structure composed of metal struts, the actual contact surface area between the metal foam and metal tube is relatively small. Consequently, many gaps are created between the metal foam and the metal tube. Such gaps may create numerous problems including a reduction in heat transfer efficiency and strength. Further, when using a braze material such as foils, it is difficult to maintain the foil positioning when the metal foam wrapped in foil is inserted into the metal tube. The resulting assembly may have insufficient braze material in portions of the joint to sufficiently wet the metal foam and metal tube and therefore join the metal foam to the metal tube. These problems are surprisingly solved by the present invention.